In brake systems equipped with electrical braking control, the braking power transmitters produce a corresponding electrical output signal, which is then used for control of the brake cylinder pressure. In this case, the brake cylinder of the vehicle is fed compressed air by means of solenoid valves (modulators) which can be controlled by either an analog or a digital means.
In a pneumatic braking power transmitter (pedal valve) of modern construction, the force applied by the driver's foot is transmitted via a pedal to a rubber spring with a non-linear force-distance characteristic. A pneumatic control pressure can thereby be achieved, which increases progressively with the distance or the angle of tilt of the pedal. The purpose of this measure is to make a precision braking operation possible in the initial range of pedal movement (partial braking) with a low control pressure. In the final range of pedal movement (full braking), on the other hand, the braking pressure increases to a greater extent. The braking pressure curve achieved in this manner also corresponds to the force applied by the driver's foot.
A brake pedal valve of the type described above is generally known in the art. One such brake pedal valve is described in German Pat. No. 29 37 657. On this particular known valve, there is also a potentiometer, which is used to control a retarder and to emit specified electrical switching signals.
A disadvantage with the use of a rubber spring is that its action is a function of the temperature, and it is also subject to the effects of aging.
These disadvantages are not experienced when steel springs or metal springs are used. In the case of steel or metal springs, however, the desired progressivity of the brake pressure modulation can only be achieved in a complex manner, for example, by parallel connection of several different springs, or by a spring with an increasing wire thickness.
German Pat. No. 19 61 039 describes a braking power transmitter, in which the force exerted on a brake pedal by the driver's foot is transmitted by means of a steel spring to a power measurement apparatus. The power exerted is converted into an electrical value in the power measurement apparatus, this value is used by an electronic control apparatus as a setpoint value. The actual value of braking pressure acting on the brake cylinder of the vehicle is set proportional to the setpoint value. The braking force therefore increases in a proportional, or linear manner, as a function of the force applied by the driver's foot.